Twin-type slewing cranes are used for handling loads on board ship, the individual cranes which together make up the twin cranes being either used singly to load and unload individual holds, or in combination for moving heavy freight.
Ordinary twin slewing cranes have a large mass (weight) and are expensive as well, since each single crane is constructed and arranged in such a way as to allow an individual slewing motion. Each crane is thus rotatably arranged on a common platform which itself can be rotated round a foundation firmly anchored in the hull of the ship. Such cranes have three slewing rims, three gear rings, three slewing mechanisms and, in addition, a large common platform. This type of construction makes for complex and expensive assembly procedures and leads to high costs of transport, assembly, inspection and maintenance.
Another type of twin slewing cranes using only a single gear ring and a slewing rim is sometimes met with. This rim, or bearing, however, is constructed so as to provide a double pivoting link, that is, two bearing functions are combined so that the middle race of the bearing is attached both to the foundation of the crane and to the gear ring, while the outer race of the bearing is connected by a collar arm to one crane, and the inner race is connected, by another collar arm, to the other crane. This type of bearing is very special, and since it produces a slewing rim of very large diameter it is extremely expensive and difficult to repair or replace if it is damaged. In addition to this, the positions of the collar arms in relation to the bearing require different mechanical constructions in each crane while at the same time the slewing mechanisms in each crane must be mounted differently, in one the driving motor is mounted upwards, in the other it is mounted downwards. Disadvantages are complications of construction and assembly of the mechanism and the fact that, e.g. the driving motors must be differently designed to allow drainage of condensing water.